A controlled feeding study from South Dakota State University, Brookings, showed that older adults who ate fewer ultra-processed foods (UPFs) naturally consumed fewer calories, lost weight and abdominal fat, and showed improvements in insulin, nutrient-sensing hormones, and inflammation.

“Counting nutrients is not enough,” Moul Dey, professor of health and nutritional sciences at the university, said. “The degree of processing changes how the body handles those same nutrients. Diet quality depends not only on nutrients but also on the ingredients and the level of processing, considered together.”

UPFs are industrial products made by reconstructing parts of whole foods with synthetic additives such as flavors, colors, preservatives, and emulsifiers.

Older adults completed an 18-week feeding study with two diet periods of eight weeks each, separated by a short break of at least two weeks. Meals were designed and administered by the university’s human nutrition research team, prepared by a professional local chef, and eaten at home by participants to reflect everyday eating patterns.

Every meal and snack was fully prepared and provided to eat at home. One diet was meat-based, featuring pork as the main protein source, and the other was plant-based, centered on lentils, beans and peas. Both followed the nutrient goals of the Dietary Guidelines for Americans.

On average, participants spontaneously reduced calorie intake and experienced about 10% total body fat loss and 13% belly fat loss on both diets, along with a 23% improvement in insulin sensitivity and favorable changes in inflammatory markers and nutrient-sensing hormone levels. Daily calorie intake decreased by roughly 400 calories per day, even without instructions to restrict calories.

These results suggest that replacing UPFs with minimally processed ones can enhance metabolic efficiency and body composition in older adults, within balanced, US–aligned diets.

Researchers note that the 18-week trial included a small sample of 36 participants who completed the study, and that larger studies are needed to confirm long-term outcomes.

At the one-year follow-up, when participants’ UPF intake had gradually increased again, many of the metabolic improvements observed during the trial diminished, suggesting that benefits depend on sustained reductions in UPFs. Still, the consistency of effects across both diet patterns underscores the central role of food processing in metabolic health.

“This study moves past the usual debate over whether plant-based or animal-based diets are better,” Dey said. “Both can be health-promoting when foods are simply prepared and nutritionally balanced.”

To access the full article, published in Clinical Nutrition, click here